1. Field of the Invention
The present invention relates to a process for manufacturing polyethylene and a plant for conducting said process.
2. Description of Related Art
Polyethylene, in particular low density polyethylene (LDPE) is in general manufactured in a high pressure process using a reactor such as a tubular reactor. The exothermic polymerization reaction is carried out under operating pressures between 500 and 4000 bar and temperatures between 165 to 340° C. The polymerization of ethylene is started by free radical initiator, usually using peroxides or oxygen.
These processes are highly integrated processes requiring complex plant networks. A LDPE production plant comprises usually one or multiple compressor units for compressing the ethylene feed, a preheater for pre-heating the ethylene feed and opt. other substance, a tubular reactor for the actual polymerization process of the ethylene feed coming from the preheater, a high pressure separator (HPS) for separating the polymer monomer mixture leaving the tubular reactor and a low pressure separator (LPS) for further separation of the polymer monomer mixture. The molten polymer is the passed from the LPS to a finishing section including an extruder. The monomer phase comprising ethylene as main component is usually recycled to the ethylene feed entering the compressor units (U.S. Pat. No. 6,596,241 B1, US 2005/0192414 A1).
Low density polyethylene is characterized by a relative high number of short side chains. The short chain branches regulate the flexibility and thus density of the polymer. An increased number of short chain branches improves the flexibility and optical properties, but reduces the mechanical strength. The melt strength in turn is influenced by the molecular weight tail and number of long chain branches.
In order to regulate the molecular weight so called chain transfer agents are added to the ethylene feed. These chain transfer agents promote the transfer of a growing polymer chain to another molecule thereby reducing the average molecular weight of the final polymer.
Besides chain transfer agents comonomers can be added to the ethylene feed in order to modulate the final polymer properties. It is for instance desirable to obtain polymers, which are able to be crosslinked following the melt forming process of its final product. Crosslinked polyethylenes are for instance extensively used for wire and cable applications or pipes.
Crosslinking can be carried out by for instance adding free radical forming agents such as peroxides to the polymer composition prior extrusion of the cable or pipe. The crosslinking is initiated by heating in a subsequent vulcanisation step in a manner that peroxide is decomposed under formation of free radicals.
Another possibility for crosslinking is the introduction of hydrolysable silane groups into the polymer. In this case crosslinking is carried out by moisture curing wherein in a first step the silane groups are hydrolysed resulting in the formation of silanol groups, which in a second step are crosslinked by a condensation reaction releasing water.
Processes for producing polyethylene-silane copolymers are known. EP 1 923 404 B1 describes a method wherein ethylene and an unsaturated silane compound are polymerised at a pressure of 100-400 MPa and at a temperature of 80-350° C. in a multi-zone reactor comprising two or more reaction zones, wherein more than 60 wt % silane compound are added to the first reaction zone of the reactor. The feeding of the majority of the silane compound into the first reaction zone of a reactor allows for an increased conversion of the silane comonomer. In this case the stream containing the unsaturated silane compound was compressed in a compressor before entering the reactor.
A general problem during the production of polyethylene-silane-copolymers is that during its production the volatile silane comonomer such as vinyltrimethylsilane (VTMS) is able to leave the plant equipment such as the compressor unit of a plant through wear and tear of the sealings and stuffing boxes to the motionwork where abrasive silanol particles are formed leading to increased need for maintenance and downtime of the hypercompressor including vibrations.
Another side effect of silane comonomers is that they premature polymerise already during compression causing plugging of the intercooler and preheater piping. It also has a negative impact on the lubrication of the movable part of the compressor as the silane influences the viscosity of the lubricant as well as its adhesion forces to metals. Furthermore, the high silane concentration in the leakage gas/purge gas requires a sophisticated treatment of this stream to separate the silane prior to further processing